These will be used by Chromium's crypto::ECPrivateKey to work with
EncryptedPrivateKeyInfo structures.

Note this comes with a behavior change: PKCS8_encrypt and PKCS8_decrypt
will no longer preserve PKCS#8 PrivateKeyInfo attributes. However, those
functions are only called by Chromium which does not care. They are also
called by the PEM code, but not in a way which exposes attributes.

The PKCS#12 PFX code is made to use PKCS8_parse_encrypted_private_key
because it's cleaner (no more tossing X509_SIG around) and to ease
decoupling that in the future.

crypto/pkcs8's dependency on the legacy ASN.1 stack is now limited to
pkcs8_x509.c.

BUG=54

Change-Id: I173e605d175e982c6b0250dd22187b73aca15b1a
Reviewed-on: https://boringssl-review.googlesource.com/14215
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>